1. Field of the Invention
The present invention relates to a sheet feeding device and an image forming apparatus and, more particularly, to the one in which sheets are separated and fed by blowing air to the sheets.
2. Description of the Related Art
Conventionally, image forming apparatuses such as printers and copying machines are provided with a sheet feeding device of feeding sheets one by one from a sheet containing portion in which a plurality of sheets are contained. There is such a sheet feeding device of air sheet feeding type in which air is blown to the end portion of a sheet stack contained in a sheet containing portion to blow up several sheets, and only one sheet is sucked to a sucking and conveying belt disposed thereabove to be conveyed. For example, a sheet feeding device of this type is disclosed in Japanese Patent Application Laid-Open No. H07-196187.
FIG. 14 illustrates one example of a sheet feeding device of such air sheet feeding type. As illustrated in FIG. 14, a tray 12 on which sheets S are stacked is disposed so as to be capable of being lifted or lowered in a storage 11, being a sheet containing portion in which a plurality of sheets S is contained. Moreover, there are provided above this storage 11, a conveying portion 50A that sucks and conveys sheets S, and an air blowing portion 30 for blowing air to the end portion of a sheet stack on the tray to cause several sheets S to blow up, as well as to separate them from one another.
Herein, the conveying portion 50A is provided with a sucking and conveying belt 21 that is passed over belt driving rollers 41, and sucks sheets S to convey them rightward in FIG. 14, and a suction fan 36 generating a negative pressure for causing a sheet S to be sucked to the sucking and conveying belt 21. Furthermore, there is provided a suction duct 51 disposed inside the sucking and conveying belt 21, and acting to suck in air through suction holes formed in the suction belt 21. In addition, to make ON/OFF of sucking operation made by the suction fan 36, there is provided a suction shutter 37 disposed between the suction fan 36 and the suction duct 51.
Furthermore, the air blowing portion 30 is provided with a loosening nozzle 33 and a separation nozzle 34 for blowing air to the upper portion of a contained sheet stack, a separation fan 31, and a separation duct 32 supplying air from the separation fan 31 to each of the nozzles 33 and 34.
Further, a part of air having been sucked in the direction indicated by the arrows C with the separation fan 31 is passed through the separation duct 32 to be blown in the direction indicated by the arrows D with the loosening nozzle 33, and acts to blow up several upper sheets of the sheet stack supported on the tray 12. Moreover, other air is blown in the direction indicated by the arrows E with the separation nozzle 34, and acts to separate only the uppermost sheet one by one out of the several sheets blown up with the loosening nozzle 33 to be sucked to the sucking and conveying belt 21.
Incidentally, to make a sheet S to be sucked to the sucking and conveying belt 21 like this, an uppermost sheet Sa of the sheet stack contained in the storage 11 needs to be maintained in a predetermined sheet feeding position capable of being sucked to the suction belt 21.
Accordingly, conventionally there has been provided a sheet surface detecting mechanism formed of a sheet surface detecting sensor and sensor flag acting to detect positions of the uppermost sheet Sa. In this sheet surface detecting mechanism, lifting and lowering of a tray 12 supporting sheets are controlled by detecting the displacement of the sensor flag with the sheet surface detecting sensor. Further, conventionally, the sheet surface detecting sensor and the sensor flag of such a sheet surface detecting mechanism are disposed in an internal part of the suction duct 51. For example, one example of this construction is described in Japanese Patent Application Laid-Open No. 2003-95467.
However, when a sheet surface detecting sensor and the like are disposed in the suction duct 51 like this, there has to be a space for containing the sheet surface detecting sensor and the like in the suction duct 51. Moreover, when such containing space is formed, a suction duct 51 becomes large, and thus the whole of an image forming apparatus comes to be larger accordingly.
Further, the capacity of a suction duct 51 comes to be larger. Herein, since the capacity of the suction duct 51 is closely related to the power of a suction fan 36, a larger capacity of the suction duct 51 leads to upsizing of the suction fan 36, resulting in waste of energy consumption or higher costs. In addition, although the suction duct 51 is required to have a high air-tightness, it is significantly difficult that a sheet surface detecting mechanism is smoothly operated, as well as electrical parts such as sensors of the sheet surface detecting mechanism are disposed while keeping air-tightness.
For these reasons, to achieve downsizing of an apparatus or maintain a stable performance, as illustrated e.g., in FIG. 15, it is practical that a sheet surface detecting mechanism 50B is disposed outside a suction duct 51. Now, the sheet surface detecting mechanism 50B disposed outside the suction duct 51 like this, and functioning to detect the upper surface of sheets S stacked on a tray 12, is described.
This sheet surface detecting mechanism 50B is provided with a sheet surface detecting sensor flag 52 pivotally supported about a support shaft 53, and contacted with the upper surface of sheets S, and a first sheet surface sensor 54 and a second sheet surface sensor 55 made to be ON/OFF by turning of the sheet surface detecting sensor flag 52.
Herein, the sheet surface detecting sensor flag 52 is provided with a contact portion 52A in contact with the upper surface of the uppermost sheet Sa, a first detecting portion 52B shading a light-receiving portion of the first sheet surface sensor 54, and a second detecting portion 52C shading a light-receiving portion of the second sheet surface sensor 55.
In the sheet surface detecting mechanism 50B of such construction, when the tray 12 is lifted for feeding sheets S, the contact portion 52A of the sheet surface detecting sensor flag 52 is in contact with the upper surface of the uppermost sheet Sa, and thereafter the sheet surface detecting sensor flag 52 is pivoted accompanied by the rise of the tray 12. Then, when the sheet surface detecting sensor flag 52 is pivoted like this, the first detecting portion 52B and the second detecting portion 52C make ON/OFF of the first sheet surface sensor 54 and the second sheet surface sensor 55 as appropriate respectively.
Furthermore, a controller acting to control lifting and lowering of the tray 12 makes lifting and lowering of the tray 12 based on ON/OFF of these first and second sheet surface sensors 54 and 55 to maintain the uppermost sheet Sa in a predetermined sheet feeding position.
However, in conventional sheet feeding devices and image forming apparatuses provided with such a sheet surface detecting mechanism, for example, in the case of sheets which end portions of the downstream side in a sheet conveying direction are curled upward, when air is blown to the sheets from the loosening nozzle 33, they will be in such a blown-up state as illustrated in FIG. 16. Herein, in this state, while the sheet surface height of the uppermost sheet Sa in a position where the contact portion 52A of the sheet surface detecting sensor flag 52 is in contact, is optimum (for example, SL), the downstream side end portions of sheets S are contacted with the sucking and conveying belt 21.
Then, in such a state, when the uppermost sheet Sa is sucked to the sucking and conveying belt 21, as well as a separating air is blown from the separation nozzle, the separating air indicated by the arrow is interrupted with curls of the sheets not to be capable of smoothly coming in between the sheets. Thus, sheets cannot be separated (loosened) sufficiently from one another.
Consequently, the next sheet Sb or the subsequent plural sheets of a sheet stack are conveyed erroneously in association with the uppermost sheet Sa, thus leading to a problem of the occurrence of double feed of sheets or jamming (sheet jamming).
That is, in the case where a sheet surface detecting mechanism 50B is disposed outside of the suction duct 51 for the purpose of preventing upsizing of apparatuses, for example, in case of sheets curled upward, the distance of sheets with respect to the sucking and conveying belt 21 cannot be exactly recognized. As a result, feeding failures such as double feed of sheets or jamming will occur.